1. Field of the Invention
The present invention relates to a centrifugal casting die for casting a thin-walled cylindrical resinous material and a method for manufacturing the casting die as well as the casting material by using the same, a blade used in an electrophotographic apparatus and a method for manufacturing the blade.
2. Description of the Related Art
A schematic structure of a conventional electrophotographic apparatus is shown in FIG. 7, which carries out a contact-charging process and a contact-transfer process without using a corona discharge. That is, an electro-photosensitive drum 101 on which an electrostatic latent image is formed is provided with an electroconductive basic layer made, for example, of aluminum and a photoelectric-conductive layer formed on the outer surface of this electroconductive basic layer. A charging roller 102 which is in contact with the outer surface of the electro-photosensitive drum 101 and uniformly charges the latter at a predetermined electric potential level is provided with a core spindle 102a and an electroconductive elastic layer 102b encircling the core spindle 102a. This charging roller 102 is pressed upon the outer surface of the electro-photosensitive drum 101 by biasing means such as a spring (not shown), and is made to passively rotate as the electro-photosensitive drum 101 is driven. By applying a bias voltage which is a direct current integrant added with an alternate current integrant, or solely a direct current integrant to the core spindle 102a of the charging roller 102, the surface of the electro-photosensitive drum 102 is contact-charged to a predetermined electric potential level. By exposing the surface of the electro-photosensitive drum 101 charged at the predetermined electric potential level as describe above with image information via a focusing lens 103 of an exposure device using a laser or LED, a static latent image is formed on the surface of the electro-photosensitive drum 101 in correspondence to this image information, and is visualized as a toner image by a toner 105 supplied via a developer sleeve 104a of a developing device 104. A developer blade 104b is brought into slide-contact with the outer periphery of the developer sleeve 104a, for properly restricting an amount of the toner 105 to electro-photosensitive drum 101. A transfer roller 106 having a core spindle 106a supported at opposite ends to be rotatable and a cylindrical electroconductive elastic layer 106b encircling the core spindle 106a charges the toner image formed on the surface of the electro-photosensitive drum 101 from a back surface of a transfer medium 107 such as paper to have a polarity reverse to that of the toner 105, whereby the toner image on the electro-photosensitive drum 101 is transferred to the surface of the transfer medium 107. The transfer medium 107 on which the toner image is transferred is separated from the electro-photosensitive drum 101 and nipped between a pair of fixing rollers 108 whereby the toner image is fixed to the transfer medium 107 by heat and pressure. Residual toner left on the electro-photosensitive drum 101 is wiped off therefrom by a cleaning blade 109a in a cleaner 109. Thus, the surface of the electro-photosensitive drum 101 is cleaned.
Various blade members used in such an electrophotographic apparatus, for example, the above-mentioned developer blade 104b for properly restricting an amount of toner 105 carried on the outer periphery of the developer sleeve 104a or the cleaning blade 109a for wiping off the residual toner 105 from the surface of the electro-photosensitive drum 101 are manufactured by casting thermosetting resin excellent in resistance to abrasion or in mechanical properties such as urethane rubber into the interior of a cylindrical die for a centrifugal casting rotating at a high speed so that the resin is spread over the inner surface of the casting die to form a thin layer which is then heated to be a cylindrical sheet material (in a case of the developer blade 104b, the thickness is in a range from 0.5 to 2 mm, while in a case of the cleaning blade 109a, from 1.5 to 3 mm), and after removing the cylindrical sheet material from the casting die, cutting the same to be pieces having a predetermined size and shape which pieces are adhered to a metallic mounting member 104c or 109b via an adhesive.
An appearance of the conventional die for a centrifugal casting to obtain such a thin-walled resinous cylindrical material is schematically shown in FIG. 8 in a partly broken manner. That is, a cup-shaped die body 110 has an opening 110a at one end and a rotary shaft 110b at the other end which projects outward and is coupled to a drive source (not shown). A thermosetting resin to be fed to the inner surface 110c of the die body 110 via the opening 110a is uniformly spread all over the inner surface 110c by the rotation of the die body 110 driven by a drive source and hardened by receiving heat generated from a heater (not shown) embedded in the die body 110. A thin-walled centrifugal casting is released from the inner surface 110c, i.e., an inner wall, of the die body 110 after the rotation of the die body 110 has stopped, and cut into pieces of a predetermined size and shape, which are then used as the developer blade 104b or the cleaning blade 109a as described above.
In this regard, an inner surface 111a of the thin-walled resinous cylindrical material 111 is referred to as an air surface, while an outer surface 111b opposite thereto to be in contact with the inner wall 110c of the die body 110 is referred to as a casting surface. The air surface 111a is a mirror surface but the casting surface 111b has a surface roughness transferred from the inner wall 110c of the die body 110. Thus, when the thin-walled resinous cylindrical material 111 manufactured by the centrifugal casting is used as a developer blade 104b or a cleaning blade 109a, the air surface 111a generally excellent in smoothness is used as a surface to be in slide-contact with the developer sleeve 104a or the electro-photosensitive drum 101.
While the thin-walled resinous cylindrical material manufactured by the centrifugal casting can easily have a uniform thickness, the requirement for the further precision of the size and shape of the blade has recently been increased because the speed of an electrophotographic apparatus such as a printer or a copier has been more accelerated and the image quality thereof has become higher.
When a center axis of the rotary shaft 110b is eccentric from a center axis of the inner surface 110c of the conventional die body 110 shown in FIG. 8, or the deviation from circular form of the inner surface 110c of the die body 110 is low, vibration occurs in the inner surface 110c of the die body 110 during the rotation of the casting die, causing the uneven distribution of centrifugal force in the circumferential direction thereof and results in an inconvenience in that a wall thickness of the obtained thin-walled resinous cylindrical material 111 lacks the uniformity in the circumferential direction.
To solve such an inconvenience, a proposal is disclosed in Japanese Patent Application Laid-Open No. 2000-172068 in that resin for compensating for the eccentricity of the inner surface of the die body is supplied in advance to the inner surface of the die body and heat-set to form an eccentricity-compensation layer integral with the die body, after which resin for forming the thin-walled resinous cylindrical material is supplied to the inner surface of the eccentricity-compensation layer. According to this method, since the eccentricity of the rotary axis of the inner surface of the eccentricity-compensation layer is automatically corrected relative to the center of the rotary shaft of the die body during the casting process, it is possible to almost eliminate the variation of the circumferential directional-wall thickness of the thin-walled resinous cylindrical material cast on the inner surface of the eccentricity-compensation layer. In addition, since the inner surface of the eccentricity-compensation layer becomes a mirror surface without the mirror finishing, it is possible to finish the casting surface of the thin-walled resinous cylindrical material thus obtained as a mirror surface similar to the air surface thereof. Particularly, when the silicone rubber is used as the eccentricity-compensation layer, the thin-walled resinous cylindrical material is easily released from the eccentricity-compensation layer without coating a die-lubricant on the inner surface of the eccentricity-compensation layer, whereby it is possible to reduce the man-hour or the production cost for the rinsing process or others due to the elimination of the die-lubricant.
When the casting of the thin-walled resinous cylindrical material has been repeated by this method, however, the releasability of the inner surface is gradually lowered, which necessitates the removal of the eccentricity-compensation layer from the inner surface of the die body after the repetition of a certain number of casting cycles and then the formation of a fresh eccentricity-compensation layer. Particularly, when the silicone rubber having the deterioration in releasability is removed from the inner surface of the die body, the silicone rubber is liable to be torn and left on the inner surface. The complete removal thereof from the inner surface of the die body necessitates an extremely large amount of man-hours.
Also, according to the method disclosed in Japanese Patent Application Laid-Open Nos. 2000-172068 and 2001-305858, since the inner surface of the eccentricity-compensation layer has a mirror surface even though the inner surface of the die body is not subjected to the mirror finishing, the casting surface of the resultant thin-walled resinous cylindrical material is finished as a mirror surface similar to the air surface thereof to increase the tackiness; i.e., stickiness. As a result, when the obtained thin-walled centrifugally casting material is processed to a blade or others of the electrophotographic apparatus, it is liable to adhere to a casting jig or others and difficult to be handled to lower the workability.